I've been running F5 for close to 4 months every day, the sound has change several times over that period and only in last month has it good to where id called burnt in, during that period I've tried different types of speaker cable and interconnects, so if you want sibilance go to QED silver Anniversary's, once its burnt in I think you will find Ant-cables work best. You could put some 8 - 10 ohm resistors in place of speakers and let it run 24/7. But leave it 2 or 3 months before final adjustments.

and Thank you Nelson, and my neighbours thank you, Music Reference valve pre, 91db Tannoys, means plenty of volume. thanks to all contributors as well.

This has all what you'd expect from a moderately high end amp, so its worth the rest of your system being up to that standard to hear it at its best, and other High Fi buffs that have listened agree.There's been nothing fancy added like extra mosfets etc, its just as the master designed it.

I am building my first F5 exactly to the masters original design (updated w/22K current limiters) with IRFP240/FQA12P20, but I have a set of Toshibas for a second set of monoblocks. I was not going to use them unless a saw some well-explained mods to the original circuit, or, a revised schematic. Prooptiki's schematic matches my notes for the most part. I plain to do direct A/B comparisons when finished.

I use Toshiba outputs, sound wonderful, and w/o tempco they are very DC stable.
I think they are a bit more detailed than Nationals, while not sounding peaky or rough.
They are smooth and refined. I use .47r, but lowered R3 and R4 to 1K to get a little more
'action' out of the 5K pot. They otherwise bias up very quickly.

As posted here ages ago, my preferred configuration is Toshiba 2SK1530 / 2SJ201, balanced,
2A bias, 0R22 source resistors. I like Toshibas better than Fairchilds, but I am known to be biased.

> Am I right in thinking that we'd need to see ~2.7V across R3/R4 (instead of the 4.2V for
the IR or Fairchild parts) in order to bias the Toshibas to 1.3A?

Yes, so you need to adapt the circuit (e.g. lower bias for the JFETs).
I used 6mA Idss, which are unfortunately not easy to get. You can of course also lower the resistor value
at the drain of the JFETs.

i only reduced the trimmer value and got 2,7 volts for 2sk1530/j201..and everything is allright..
i didnt change anything other..

I got these Wild Burro 8" fullrange and my plan is to mate them with a 35mm TB, in a floor based omni design
In my experimental setup the small TB sounds pretty good, with lots of resolution, when carefully adjusted
When I get around to build my F5 I will know if its a good match
Maybe next year

__________________sometimes we know very little, and sometimes we know too much

I thought this is the key too... Now the last cap is small siemens sikorel,but i think good MKP should have positive influence,as i know allready, with these a-class transistor amps ANY mod in PSU are noticable.
I really hope that F5 can give suzuki superbike like performance with my titan dioxide tweeters,more "being there" felling.Now it's just amp that dont involve me much in the music.With harsh cd,speakers,maybe it would be very good.

I would also like to share that in my set up , the addition of a pre transforms the sound for lethargic, lacking drive to' being there " with the musician experience . The treble in my ribbon tweeter is very detail and precise . The varying force of the drummer on the hat could be appreciated.

The sound of the F5 can vary all over the map. NP already said that the bias setting alone will change things.

Bias is important, but also the F5 depends on a specific "sweet
spot" or nulling effect where the positive and negative halves
are just right. It takes a distortion analyzer to find it quickly
or a very good ear to find it slowly. Go off the balance and it
fattens right up, and some like that.

Personally I find it to be partly a function of the loudspeaker
driver. With something like the Feastrex D9nf, a little fat
doesn't hurt, or you can switch to an F3, or perhaps a nice
glass of cabernet.